Air cooled centrifugal liquid chillers are known in the art, as exemplified by U.S. Pat. No. 3,857,253. Moreover, the problem associated with chillers of this type when exposed to low ambient conditions during machine shut down have been recognized and a solution thereto proposed in U.S. Pat. No. 4,081,971. In this patent, a relatively complex valve is provided in the conduit means which connect the air cooled condenser to the evaporator, which valve is automatically closed during those times when the chiller is inoperative so as to prevent refrigerant flow to the evaporator. The present invention represents an improvement over this patent through the provision of a trap disposed within the conduit means connecting the air cooled condenser and evaporator in order to prevent refrigerant flow therebetween during those times when the chiller is inoperative. The present invention has been found to present a cost savings while also affording increased reliability of operation over that scheme disclosed in the aforementioned patent.